POLARISATION OF LIGHT. 



47 



have sometimes the very same tint, and 

 they exhibit no peculiarities in their ab- 

 sorptive properties when exposed to 

 polarised light. 



Some of the preceding crystals exhi- 

 bit different colours, even when common 

 light is transmitted in directions parallel 

 and perpendicular to their axes. A spe- 

 cimen of sapphire had a deep blue co- 

 lour in one direction and a yellowish 

 green in the transverse direction. Tour- 

 maline is often green along the axis, and 

 of a deep red in a transverse direction ; 

 and Mr. Herschel has mentioned a va- 

 riety of sub- oxysulphate of iron, which 

 is of a light green colour across the 

 axis, while in the direction of the axis 

 its colour is a deep blood red, so intense 

 that a thickness of l-20th of an inch 

 scarcely allows any light to pass. The 

 potash * muriate of palladium was ob- 

 served by Dr. Wollaston to be of a deep 

 red along the axis, and a vivid green in 

 a transverse direction. 



The phenomena of absorption in crys- 

 tals with two axes are extremely inte- 

 resting. The two colours which appear 

 in the ordinary and extraordinary pen- 

 cils are seen diverging from the resultant 

 axes when exposed lo common light, 

 as shown in/g\ 51, which represents the 

 phenomena in Mite or Dichroite. 



ftr.M. 



This mineral crystallises in six or 

 twelve-sided prisms, which appear of a 

 deep blue colour when seen along the 

 axis, and of a yellowish brown when 

 seen in a direction perpendicular to the 

 axis of the prism. 



If abed, Fig. 52, is a section of a 

 prism of iolite by a plane passing through 

 the axis of the prism, the transmitted 

 light will be blue through the faces a b, 

 and d c, and yellowish brown through 

 ad, be, and in every direction peipendi- 

 cular to the axis of the prism. If we 

 grind down the angles, a, c, b, d so as to 



replace them with faces m n, m' n', and 

 op, o'p', inclined 31 41' toad, or to 

 the axis of the prism ; then, if the plane 

 abed passes through the resultant axis 

 of double refraction, we shall observe, 

 by transmitting polarised light through 

 the crystal in the directions a c, b d, and 

 subsequently analysing it. a system of 

 rings round each of these axes. The 

 system will exhibit the individual rings 

 very plainly if the crystal is thin ; but if 

 it is thick, we shall "observe, when the 

 plane abed is perpendicular to the 

 plane of primitive polarisation, some 

 branches of blue and white light, di- 

 verging in the form of a cross Jrom the 

 centre of the system of rings, or the 

 poles of no polarisation, as shown at 

 p and p', fig. 51, where the shaded 

 branches represent the blue ones. The 

 summits of the blue masses at p and fl 

 are tipped with purple, and are sepa- 

 rated by whitish light in some speci- 

 mens and yellowish light in others. 

 The white light becomes more blue from 

 p andjo' to o, where it is quite blue, and 

 more yellow from p and p to c and d, 

 where it is completely yellow. When 

 the plane abed is in the plane of pri- 

 mitive polarisation, the poles p p' are 

 marked by spots of white light, but 

 everywhere else the light is a deep blue. 

 In the plane cadb,fig. 52, the mi- 



Ftg. 52. 



neral, when we look through it at com- 

 mon light, exhibits no other colour but 

 yellow, mixed with a small quantity of 

 blue, polarised in an opposite plane. 

 The ordinary image at c and d is yellow- 

 ish brown, and the extraordinary image 

 faint blue ; the former acquiring some 

 blue rays, and the latter some yellow 

 ones from c and d to a and b, where the 

 difference of colour is still highly marked. 

 From a and b towards p and p' the yel- 

 low image becomes fainter, till it changes 

 into blue, and the weak blue image is 

 reinforced by other blue rays till the in- 

 tensity of the two blue images is nearly 

 equal. The faint blue image increases 

 in intensity as the incident ray ap- 

 proaches from c and d to p and p', and 



